Not long ago, many scientists might have picked B, notes Gene Carl Feldman, project manager for an orbiting imager, known as SeaWiFS. It's giving researchers amazing new views of the planet's biological activity.

Now, however, the 3-year-old imager is showing how critical ocean-dwelling phytoplankton are to the global carbon cycle. In the process, it's giving scientists a powerful new tool for tracking the impact of climate change.

From 1997 to 2000, a record El Nino began to relax its grip on climate. As La Nina's hand took over, the globe's greenery increased its carbon uptake from 111 billion tons to 117 billion tons, according to the satellite data. The change was most pronounced in the tropical Pacific.

Ocean circulation along the South American coast shifted, bringing more nutrients from the deep ocean to nourish plantlike phytoplankton at the surface.

The tropical Pacific shifted from a photosynthetic "desert similar to the Sahara, to a virtual rainforest, over the course of a few weeks," says Dr. Feldman, who works at the Goddard Space Flight Center (GSFC) in Greenbelt, Md.

Land-based plants showed regional changes in response to the shift from El Nino to La Nina, he adds. But globally, land plants contributed little to the increased uptake.

"The powerhouse behind carbon fixation is in the oceans," says Michael Behrenfeld, an oceanographer at the GSFC and the lead author of a paper reporting the results

in the current issue of the journal Science. Past satellite data suggested that 40 percent of the Earth's photosynthetic activity occurs in the oceans. SeaWiFS data suggest the figure is more like 50 percent, he says.

SeaWiFS is a high-tech version of a camera's light meter, Feldman explains. From an altitude of 705 kilometers (437 miles), the imager records the amount of light reflected from the oceans and land in eight "color" bands along the electromagnetic spectrum. "Based on some serious number crunching, you can relate color to the level of photosynthesis," he says.

Such data have been available for land activity for at least 20 years, the researchers say. But similar information on ocean "primary production" has been spotty in coverage and quality.

SeaWiFS has changed that. It gathers in one minute data that would take an oceanography vessel 10 years to gather. It also gathers information uniformly, allowing for more accurate accounting of the role plant life plays in the carbon cycle in each venue.

"One of the great uncertainties regarding climate change is how biology will respond," says Jorge Sarmiento, a Princeton University climate researcher, referring to the plant and microbial life that act as sinks for CO2. "These data are essential to projecting the future trajectory of CO2."